My invention relates to high fidelity phonograph preamplifier circuits used to amplify the outputs of magnetic cartridge phonograph pickups. More specifically, my invention relates to such a unique preamplifier circuit which substantially eliminates distortion present, but not heretofore measured, in high quality preamplifiers because they were tested with inadequate test signals.
My invention substantially minimizes or eliminates amplitude overload distortion, slew overload distortion and a form of distortion only identified in recent years and referred to as transient intermodulation distortion. Slew overload distortion is created when a signal fed to an amplifier stage changes more rapidly than the ability of the same to respond to the rapid change. Amplitude distortion occurs when the signal fed to the stage has an amplitude which drives the amplifier into a saturated state so as to clip the applied signal. Transient intermodulation distortion is generally caused by the inability of a negative feedback amplifier when presented with a fast changing input signal to feedback a needed feedback component in time to interact with the fast input signal. In such cases, the absence of any negative feedback at the inception of such a fast signal causes the first amplifier stage to be driven into distortion or saturation, producing amplitude clipping and/or shock-excited transient distortions in the amplifier.
Distortion in preamplifiers has heretofore generally been measured by feeding a reference sine wave of a relatively modest audio frequency of between 1 KHz and 10 KHz into the preamplifier and measuring any distortion caused thereby. In costly apparent high quality preamplifier systems for which actual low distortion measurements are obtained, it was noticed by listening to recorded music using such preamplifiers that substantial distortion was present which was not detected by the measurements referred to. The reason why the actual distortion was not measured is that the sine wave test signal used was generally inadequate to cause appreciable amounts of the above-identified forms of distortion in even apparently high quality preamplifiers, which actually produce these distortions from musical recordings having high amplitude, rapidly changing signals. I have discovered that the distortion testing signal should ideally resemble that produced at the output of a magnetic cartridge with a fast response time where the recorded signal is that obtained by a square wave of an audio frequency fed to the cutting head used to cut the record through a RIAA pre-emphasis network and with only at most a modest amount of bandwidth limiting, so that the signal from the phono cartridge has a square wave form with a high amplitude spike at the leading edge of each half cycle thereof. This test signal is believed to duplicate the heretofore distortion-producing, rapidly rising signal conditions occurring in many passages of high fidelity recorded music and reproduced by fast response magnetic phonograph pick-ups, but not heretofore measured by the conventional sine wave signal testing techniques. Thus, while the specifications for even the highest quality preamplifiers available today indicate only small levels of distortion are present in such amplifiers, there is in fact a substantial amount of audible distortion produced in these preamplifiers by these fast musical recorded passages.
While in the preprint of a paper presented to the Audio Engineering Society 1977 Convention and entitled "Phonograph Preamplifier Design Criteria Arising from System Measurements", by Holeman & Kampmann, the use of a square wave test signal to measure distortion in preamplifiers is suggested, because of the substantial bandwidth limiting utilized and the fixed modest reference amplitude, the spike square wave test signal did not have a sufficiently high peak to peak amplitude or sufficiently steep wave front to duplicate the effects of distortion producing portions of musical recordings.
To best understand the deficiencies of these prior art preamplifiers, it would be helpful first to review the common forms thereof. First of all, the high amplitude, rapidly rising, distortion-producing portions of musical recordings comprise appreciable Fourier series frequency components far beyond the audio frequency range, such as frequencies of at least about 30 KHz. The first and other stages of conventional apparently high quality preamplifiers have before feedback a bandwidth encompassing an audio frequency range below 2 KHz, the connection of the feedback circuit boosting the bandwith only to about the upper limit of the audio range, and so produce audible slew overload distortion. When negative loop feedback is used in such initial stages of the preamplifier, transient intermodulation distortion is produced when signals having high amplitude, rapidly rising wave fronts are amplified thereby. The RIAA compensation networks of these conventional preamplifiers, which includes a low frequency boost portion and a high frequency de-emphasis or rolloff portion, are commonly placed in a negative feedback loop around two adjacent high gain amplifier stages which generally compose the entire gain producing portion of the preamplifier circuit. Large amounts of negative feedback are required in such a circuit. I have discovered that when these signals are fed through such negative feedback amplifiers the avoidance of transient intermodulation distortion requires exceedingly fast response times and bandwidths like at least about 60 KHz when the feedback circuits are disconnected and preferably as much as 250 KHz which such high gain amplifier stages did not supply. Thus, the gain of the stages used in these negative feedback amplifiers is generally at least about 20-35 db, and the amount of negative feedback is generally above 30 db. This combination of high gain, limited bandwidth and large amounts of feedback at high frequencies, which results from the use of negative feedback to achieve the RIAA playback curve, guarantees substantial amounts of transient intermodulation distortion.
Another less commonly used preamplifier places the RIAA compensating networks totally in the interstage coupling path between the first and subsequent stage of the preamplifier (so that the networks are not in a negative feedback circuit. The difficulty with this design is that the low frequency boost portion of this network has a substantial insertion loss of about 20 db. This causes a noise problem if the gain of the first stage is not sufficient to overcome the 20 db RIAA network loss. This first stage of gain must handle the full system bandwidth and maximum rate of change without slew rate overload, amplitude limiting or transient intermodulation distortion. Such a design is difficult to achieve.
In addition to eliminating from a preamplifier the various forms of distortion described, it is desirable to isolate the preamplifier from impedance variations with frequency of magnetic cartridge phonograph pickups. The usual high gain first preamplifier stage in prior art preamplifiers using negative feedback did not act as a satisfactory isolation to these impedance variations of magnetic cartridge phonograph pickups.
It is an object of my invention to provide a preamplifier which does not interact with magnetic cartridge phonograph cartridge outputs producing high amplitude, rapidly rising wave fronts in a manner to create the various forms of distortion described.
Another object of my invention is to provide a phonograph preamplifier as described which is isolated from the impedance variations of a magnetic cartridge phonograph pickup.
Still another object of my invention is to provide a unique test wave form for evaluating said distortions which have not heretofore been measured by conventional distortion testing methods.